A few Pokémon fit the criteria of having the ability Infiltrator as one of its primary abilities, and being obtainable before the 6th Gym Leader. These include:
Espurr: Espurr has a chance of having this ability, along with Keen Eye.
Espurr can be found on Route 5, Hammerlocke Hills when it is not a Snowstorm or Sandstorm, and Motostoke Riverbank in Heavy Fog
Cottenee: Cottenne has a chance of having this ability, alone with Prankster.
Cottenee can be found in Stony Wilderness when it’s Overcast. The one you get via trade in Hulbury has Prankster.
Noibat: Noibat has a chance of having this ability, along with Frisk.
Noibat can be found at Bridge Field, in the air of Watchtower Ruins, and in Galar Mine #2.
In the end, I’d go with Espur when dealing with the Ice Gym, as it isn’t weak to it. In addition, it is strong against the Fighting Gym. However, Cottenee is likely best for the 7th Gym, due to Espurr’s weakness to Dark, and Cottenee strength against it.
I hope I helped! Also, sorry if someone already answered this, I spent a while on this, because I did it on my phone instead of a computer.